A conventional method which has been widely employed for producing molded products having a foamed structure comprises preliminarily foaming a styrene resin containing a foaming agent and, after allowing the preliminarily foamed resin particles to stand in air for a while, continuously charging them in a closed or open mold under reduced pressure or normal pressure, followed by heating to thereby expand and adhere to each other.
This method is advantageous for easily producing molded products having complicated shapes that are used as keep-fresh containers, cushioning packing materials, heat-insulating materials, and the like.
However, this method is limited to expansion molding of polystyrene resins and cannot be applied to expansion molding of polyolefin resins to obtain molded products of complicated shapes. The reason is that polyolefin resins are so inferior to polystyrene resins in terms of long-term preservability in the state having foamability that gases occluded in resin particles escape in a short time.
Foams of polyolefins, e.g., crosslinked polyethylene, non-crosslinked polypropylene, crosslinked polypropylene, etc., are superior to polystyrene foams in mechanical strength and oil-resistance and, therefore, have been used as a core material of a bumber, a cushioning material for packing TV sets, electric refrigerators, etc., and the like.
Known methods for molding such foamed polyolefins in a mold include the following methods:- (1) A method involving aging under pressure (hereinafter referred to as "pressure aging molding method"), in which a pressurizing inorganic gas is pressed into foamed resin particles of crosslinked polyethylene, followed by releasing the pressure to expand the resin particles to obtain resin particles having an internal gas pressure greater than 1.18 atm. that do not contract any more, and the resulting particles are charged in a mold having steam holes under normal pressure or reduced pressure and then steam-heated while keeping the pressure greater than 1.18 atm. in the inside of the particles to cause their expansion and adhesion to form a molded article, as disclosed in British Patent No. 1,445,474 (Japanese Patent Publication No. 22951/76).
(2) A pressure aging molding method, in which a pressurizing inorganic gas or mixed gas of an inorganic gas and a volatile organic blowing agent is pressed into foamed polypropylene resin particles to form particles having a total gas pressure of from 1.4 to 2.5 Kg/cm.sup.2 G in the inside thereof with partial pressures of the inorganic gas and the volatile organic blowing agent being from 0.4 to 1.1 Kg/cm.sup.2 G and from 0.8 to 1.6 Kg/cm.sup.2, respectively, and the resulting resin particles are charged in a mold having steam holes under normal pressure, followed by steam-heating to cause adhesion to obtain a molded article, as disclosed in Denish Patent No. 3,125,024 (Japanese Patent Application (OPI) No. 12035/82, the term "OPI" herein used means "published unexamined application").
(3) A method comprising applying a pressure to foamed polyolefin resin particles at a temperature of 100.degree. C. or higher to compress the particles so as to decrease the apparent volume to 90 to 40% on the initial volume, charging the compressed particles in a mold under pressure, releasing the pressure in the mold to atmospheric to cause expansion and adhesion as disclosed in U.S. Pat. No. 3,504,068, or a method comprising charging heated polyolefin foamed particles in a mold, increasing the pressure within the mold to compress the particles and, after reducing the volume of the mold, relieving the pressure within the mold to atmospheric to cause expansion and adhesion, and
(4) A method involving compression charging (hereinafter referred to as "compression charging molding method"), which comprises compressing crosslinked polyethylene foamed particles to a volume of 40 to 80% of the initial apparent volume with a pressurizing inorganic gas or volatile organic blowing agent, charging the compressed particles in a mold having steam holes, and steam-heating the particles to cause their adhesion as disclosed in Japanese Patent Publication No. 33996/78.
The pressure aging molding methods (1) and (2) are more advantageous than the compression charging molding methods (3) and (4) in that molded products having excellent appearance without voids among expanded particles, but are disadvantageous in that the pressure aging including pressing of foamed particles with an inorganic gas and pressure release to obtain foamed particles free from contraction requires a long time over 20 to 40 hours, thus reducing productivity.
The method (3) involves heating of the foamed particles outside the system and, therefore, entails cost for facilities and operation, thus giving rise to a bar to industrial application.
The compression charging method (4) is superior to the above-described pressure aging methods but requires use of highly bulky foamed particles having a small density since the foamed particles should be charged in a mold as being compressed to a volume of 40 to 80% on the initial apparent volume. As a result, transportation of the foamed particles from a maker manufacturing the foamed particles to a maker processing the foamed particles into molded products and also storage of the foamed particles entail a so much increased cost.
Further, according to the methods (3) and (4), foamed particles having a volume several to several tens times that to be charged in a mold should be compressed all at once and charged in a mold making use of a difference in pressure between a mold and a pressure-proof vessel while maintaining the atmosphere in the mold under normal pressure or under slightly elevated pressure. Therefore, it is difficult to use a mold having a complicated shape. In order to overcome this difficulty, charging may be carried out, in some cases, with a parting part between a plug and a cavity open (cracking) and, after the charging, the molding is effected in a closed mold. However, a molded article obtained by this modified method has uneven density distribution due to significantly increased density in the portion corresponding to the parting part.
Furthermore, in the above-described compression charging molding methods, when low-expansion polyethylene foamed particles (high density foamed particles) or relatively rigid foamed particles (i.e., having high compressive strength), e.g., polypropylene, etc., are used, high cost for facilities and operation would be required for compressing such foamed particles with a pressurizing gas and charging the compressed particles in a mold by pneumatic conveying, which leads to a remarkable disadvantage in carrying out the production on industrial scale.